This invention relates to a control method for electronically controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine for vehicles, and more particularly to an air-fuel ratio control method which controls the air-fuel ratio of the mixture in response to loads on the engine as well as the gear position of the transmission, thereby to optimize the driveability, emission characteristics and fuel consumption of the engine.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
On the other hand, it has conventionally been employed to lean an air-fuel mixture being supplied to an internal combustion engine for the purpose of improving the fuel consumption. Further, in order to avoid inconveniences caused by such leaning of the air-fuel mixture, such as deterioration of the emission characteristics and driveability of the engine which are derived from a drop in the conversion efficiency of a three-way catalyst for purifying the exhaust gases and a drop in the engine output, it has been proposed by Japanese Provisional Patent Publication (Kokai) No. 54-1724 to control the air-fuel ratio of the mixture in response to the speed of the vehicle or the rotational speed of the engine. However, it will be difficult to ensure required driveability of the engine merely by controlling the air-fuel ratio of the mixture in response to the vehicle speed or the engine rotational speed alone under various operating conditions of the engine as in the proposed method. Particularly, while the engine is accelerating, leaning of the mixture will result in deterioration of the driveability due to shortage in the engine output. Therefore, it is essentially required to discriminate whether or not the engine is operating in a mixture-leaning condition or in an accelerating condition.
To comply with such requirement, it has been proposed by the assignee of the present application to use engine rotational speed and intake pipe absolute pressure as paramaters for discriminating the mixture-leaning condition of the engine, and effect leaning of the mixture when the engine is operating in a predetermined low load condition where the engine rotational speed is higher than a predetermined value and the intake pipe absolute pressure is lower than a predetermined value (U.S. Pat. No. 4,445,483).
On the other hand, whether or not the engine is in an accelerating condition can be determined from the gear position of a transmission installed in the vehicle. That is, in most cases where the transmission is in a low speed gear position and the intake pipe absolute pressure in the engine is high, the driver of the vehicle wants acceleration of the vehicle, while in most cases where the transmission is in a high speed gear position, the driver wants cruising. Therefore, it will be possible to control the air-fuel ratio in a manner more appropriate to the operating conditions of the engine, if mixture-leaning regions of the engine are set in dependence on the gear position of the transmission.